When you or your coworker finish The work, saying おつかれさま is a way of recognizing the efforts of the other. It would be like a "good job!". It can be used whenever you notice the effort of others and want to show it. おさきにしつれいします is used when you have to leave early from work or a meeting. It would be like "I'm sorry to go early". いってきます is literally "I go and I come back". It's used when it is coming out. It would be like "I'm going" or "see you later".